Process for the production of liquid hydrocarbons

ABSTRACT

Process for producing normally liquid hydrocarbon products from a hydrocarbonaceous feedstock, especially from normally gaseous hydrocarbon feed, which process comprises the following steps: (a) partial oxidation of the normally gaseous hydrocarbon feed at elevated pressure using air or oxygen enriched air as oxidant, to obtain a syngas mixture comprising hydrogen, carbon monoxide and nitrogen; (b) converting hydrogen and carbon monoxide obtained in step (a) into a normally liquid hydrocarbon product and a normally gaseous hydrocarbon product; (c) separating from the reaction mixture obtained in step (b) an off-gas mixture comprising nitrogen, normally gaseous hydrocarbon product, and unconverted hydrogen, carbon monoxide and normally gaseous hydrocarbon feed, insofar as such unconverted components are present; (d) combusting at least a part of the off-gas mixture in a steam raising apparatus, producing steam of an elevated pressure; and (e) expanding the steam produced in step (d) for compressing the air or oxygen enriched air and/or the normally gaseous hydrocarbon feed used in step (a); and a plant comprising equipment in a line-up suitable for carrying out the process.

[0001] The present invention relates to a process for the production ofliquid hydrocarbons from a gaseous hydrocarbon feed, especially theoptimisation of an integrated, low-cost process for the production ofnormally liquid hydrocarbons from natural gas or especially associatedgas, at remote locations or at offshore locations.

[0002] Many publications (cf. for example WO-94/21512, WO-97/12118,WO-91/15446 and U.S. Pat. No. 4,833,170) describe processes for theconversion of (gaseous) hydrocarbon feed, such as methane, natural gasand/or associated gas, into liquid products, especially methanol andliquid hydrocarbons, particularly paraffinic hydrocarbons. Suchconversion processes may be operated at remote locations (e.g. indesserts, tropical rain-forests) and/or offshore locations, where nodirect use of the gas is possible, due to the absence of largepopulations and industries. Transportation of the gas to populated andindustrial areas, e.g. through a pipeline or in the form of liquefiednatural gas, requires extremely high capital expenditure or is simplynot practical. This holds even more in the case of relatively small gasproduction fields and/or relatively small gas production rates.Re-injection of gas into the production field will add to the costs ofthe oil production, and may, in the case of associated gas, result inundesired effects on the crude oil production. Burning of associated gashas become undesired in view of depletion of hydrocarbon sources and airpollution.

[0003] Gas found together with crude oil is known as associated gas,whereas gas found separate from crude oil is known as non-associatedgas. Associated gas may be found as “solution gas” dissolved within thecrude oil, and/or as “gas cap gas” adjacent to the main layer of crudeoil. Associated gas is usually much richer in the larger hydrocarbonmolecules (ethane, propane, butane) than non-associated gas.

[0004] Especially in view of the fact that the above-mentionedconversion processes may be operated at remote locations or at locationswhere limited space is available there is an incentive to place specialemphasis on such factors as energy and cost efficiency, compactness andcomplexity of the process or the plant in which the process is carriedout. From the references given above, however, no optimally integrated,efficient, low-cost process scheme is available.

[0005] WO-98/01514 discloses a process in which gaseous hydrocarbon feedis converted with air into syngas which, in turn, is converted intoliquid hydrocarbon product in a Fischer-Tropsch synthesis step. Asubstantial amount of the heat generated in the process is recovered andre-used in the process. Further, an off-gas mixture which is co-producedin the Fischer-Tropsch synthesis is used to fuel a gas turbine which, inturn, is used to power the compressor needed for compressing the airused in the process. The off-gas mixture in question comprisesunconverted syngas, methane by-product from the Fischer-Tropschsynthesis, and nitrogen originating from the air used. The use of air asthe oxidant in the conversion of the gaseous hydrocarbon obviates theneed of a production unit of an oxygen rich oxidant. However, thenitrogen present in the air acts in the process as a diluent gas,necessitating handling larger quantities of gas at a higher totalpressure, which requires more compression capacity.

[0006] A further disadvantageous aspect of the use of air is that thesaid off-gas mixture is diluted with nitrogen which causes that it has alow heating value. The heating value is especially low when the syngasproduction and the Fischer-Tropsch synthesis are operated efficiently,so that the content of combustible materials in the off-gas mixture isfurther decreased. In the light of the process of WO-98/01514 this willrepresent a problem when the heating value is so low that the off-gasmixture is unsuitable for use as a gas turbine fuel.

[0007] It has now been found that when the off-gas mixture is unsuitablefor use as a gas turbine fuel, sufficient energy for operating thecompressors, and even for operating the whole process, can be recoveredfrom the off-gas mixture by burning the off-gas mixture for theproduction of steam and using the steam as the source of shaft powerand/or electrical power. This finding leads to an integrated, highlyefficient, low-cost process with low capital and space requirements forthe production of normally liquid hydrocarbons from normally gaseoushydrocarbons. Further, there is-no need for the importation ofadditional fuel or other sources of energy for operating the process.The process has a high carbon efficiency, which means that a highproportion of the carbon present in the hydrocarbon feed is present inthe normally liquid hydrocarbon products.

[0008] The present finding may be applied especially when associated gasis the feedstock, which, after separation from the crude oil, is usuallyavailable at low pressure or even at ambient pressure only. The presentfinding may also be applied when the feed is gas from low pressure gasfields or largely depleted gas fields, having only a low remainingpressure. In a preferred embodiment, the process may be carried out in acompact, relatively light weight plant, making it very suitable for useon a platform or a barge, or in a dismountable plant.

[0009] A major advantage of the present finding is that relativelysimple and cheap processes and apparatus can be used. Further, anoptimal use of feedstock and energy is obtained. In a preferredembodiment an optimum carbon conversion (gas into syncrude, minimalcarbon dioxide emission), is obtained. In addition, the normally liquidhydrocarbons produced may be mixed with crude oil and transportedtogether.

[0010] The present invention thus provides a process for producingnormally liquid hydrocarbon products from a hydrocarbonaceous feedstock,especially from a normally gaseous hydrocarbon feed, which comprises thefollowing steps:

[0011] (a) partial oxidation of the normally gaseous hydrocarbon feed atelevated pressure using air or oxygen enriched air as oxidant, to obtaina syngas mixture comprising hydrogen, carbon monoxide and nitrogen;

[0012] (b) converting hydrogen and carbon monoxide obtained in step (a)into a normally liquid hydrocarbon product and a normally gaseoushydrocarbon product;

[0013] (c) separating from the reaction mixture obtained in step (b) anoff-gas mixture comprising nitrogen, normally gaseous hydrocarbonproduct, and unconverted hydrogen, carbon monoxide and normally gaseoushydrocarbon feed, insofar as such unconverted components are present;

[0014] (d) combusting at least a part of the off-gas mixture in a steamraising apparatus, producing steam of an elevated pressure; and

[0015] (e) expanding the steam produced in step (d) for compressing theair or oxygen enriched air and/or the normally gaseous hydrocarbon feedused in step (a).

[0016] The invention also relates to a plant comprising equipment in aline-up suitable for carrying out the process of this invention.

[0017] The normally gaseous hydrocarbon feed is suitably methane,natural gas, associated gas or a mixture of C₁₋₄ hydrocarbons,preferably associated gas. The C₁₋₄ hydrocarbons or mixtures thereof aregaseous at temperatures between 5 and 30° C. at 1 bara (i.e. barabsolute), especially at 20° C. at 1 bara. The normally gaseoushydrocarbon feed comprises mainly, i.e. more than 80%v, especially morethan 90%v, C₁₋₄ hydrocarbons. The normally gaseous hydrocarbon feedcomprises especially at least 60%v methane, preferably at least 75%v,more preferably at least 90%v. Very suitably natural gas or associatedgas is used, especially associated gas at a remote location or at anoffshore location. In some cases the natural gas or the associated gascomprises in addition carbon dioxide and/or nitrogen, e.g. in amounts upto 15%v or even up to 25%v of each of these compounds on the normallygaseous hydrocarbon feed.

[0018] The normally liquid hydrocarbons mentioned in the presentdescription are suitably C₄₋₂₄ hydrocarbons, especially C₅₋₂₀hydrocarbons, more especially C₆₋₁₆ hydrocarbons, or mixtures thereof.These hydrocarbons or mixtures thereof are liquid at temperaturesbetween 5 and 30° C. at 1 bara, especially at 20° C. at 1 bara, andusually are paraffinic of nature, although considerable amounts ofolefins and/or oxygenates may be present. The normally liquidhydrocarbons may comprise up to 20%w, preferably up to 10%w, of eitherolefins or oxygenated compounds. Depending on the catalyst and theprocess conditions used, also normally solid hydrocarbons may beobtained. These normally solid hydrocarbons may be formed in theFischer-Tropsch reaction in amounts up to 85%w based on totalhydrocarbons formed, usually between 50 and 75%w.

[0019] The normally gaseous hydrocarbon product comprises mainly, i.e.more than 80%v, especially more than 90%v, C₁₋₄ hydrocarbons. Thesehydrocarbons or mixtures thereof are gaseous at temperatures between 5and 30° C. at 1 bara (i.e. bar absolute), especially at 20° C. at 1bara, and usually are paraffinic of nature, although considerableamounts of olefins and/or oxygenates may be present. The normallygaseous hydrocarbon product comprises especially at least 30%v methane,preferably at least 40%v, more preferably at least 50%v. The normallygaseous hydrocarbon product may comprise up to 20%w, preferably up to10%w, of either olefins or oxygenated compounds.

[0020] Suitably, any sulphur in the normally gaseous hydrocarbon feed isremoved, for example, in an absorption tower a sulphur binding agent,such as iron oxide or zinc oxide.

[0021] The partial oxidation of the normally gaseous hydrocarbon feed,producing the syngas mixture can take place according to variousestablished processes. These processes include the Shell GasificationProcess. A comprehensive survey of this process can be found in the Oiland Gas Journal, Sep. 6, 1971, pp. 86-90. The reaction is suitablycarried out at a temperature between 800 and 2000° C. and a pressurebetween 4 and 80 bara.

[0022] Oxygen for use in step (a) is sourced from air or from oxygenenriched air. The oxygen enriched air gas comprises suitably up to 70%voxygen, preferably up to 60%v, in particular in the range of from 25 to40%v. Oxygen enriched air may be produced by cryogenic techniques, butpreferably it is produced by a process which is based on separation bymeans of a membrane, such as disclosed in WO-93/06041. The use of oxygenenriched air is not a preferred option. Preferably air is employed instep (a).

[0023] Very suitable processes for partial oxidation are catalyticpartial oxidation processes, especially as described in EP-A-576096,EP-A-629578, EP-A-645344, EP-A-656317 and EP-A-773906. In the catalyticpartial oxidation processes a catalyst bed may be applied. Suitablestructures of the catalyst bed are monolith structures, especiallyceramic foams, but also metal based structures may be used. Themonolithic structures may comprise inorganic materials of hightemperature resistance, selected from compounds of elements of GroupsIIa, IIIa, IVa, IIIb, IVb and the lanthanide group of the Periodic Tableof the Elements. Preferably the monolithic structure is zirconia based,especially stabilised zirconia. Suitable active metals for the catalyticpartial oxidation process are rhodium, platinum, palladium, osmium,iridium and ruthenium, and mixtures thereof. Preferably, rhodium and/oriridium is used.

[0024] The temperature applied in the catalytic partial oxidation isusually between 700 and 1300° C., suitably between 800 and 1200° C.,preferably between 850 and 1050° C., and the pressure is usually between4 and 80 bara, suitably between 10 and 50 bara, preferably between 15and 40 bara.

[0025] The GHSV is suitably in the range of 50,000 to 100,000,000Nl/l/h, preferably 500,000 to 50,000,000 Nl/l/h, especially 1,000,000 to20,000,000 Nl/l/h. The term “GHSV” is well known in the art, and relatesto the gas per hour space velocity, i.e. the volume of synthesis gas inN1 (i.e. at the standard temperature of 0° C. and the standard pressureof 1 bara (100,000 Pa)) which is contacted in one hour with one litre ofcatalyst particles, i.e. excluding inter-particular void spaces. In thecase of a fixed bed catalyst, the GHSV is usually expressed as per litreof catalyst bed, i.e. including interparticular void space. In that casea GHSV of 1.6 Nl/l/h on catalyst particles corresponds frequently with1.0 Nl/l/h on catalyst bed.

[0026] To adjust the H₂/CO ratio of the syngas mixture, carbon dioxideand/or steam may be introduced into the partial oxidation process. As asuitable steam source, water which is co-produced in step (b) may beused. As a suitable carbon dioxide source, carbon dioxide from theeffluent gasses of the combustion of step (d) may be used. The H₂/COratio of the syngas mixture is suitably between 1.5 and 2.3, preferablybetween 1.8 and 2.1.

[0027] If desired, a small amount of hydrogen may be made separately,for example, by steam reforming of gaseous normally hydrocarbon feed,preferably in combination with the water shift reaction, and added tothe syngas mixture. Any carbon monoxide and carbon dioxide producedtogether with the hydrogen may be used as additional feed in step (b),or it may be recycled to step (a) to increase the carbon efficiency.Alternatively, it may be combusted in step (d), together with or inadmixture with the normally gaseous hydrocarbon product.

[0028] To keep the process as simple as possible, separate hydrogenmanufacture will usually not be a preferred option. Likewise, it is nota preferred option to remove any nitrogen from the syngas mixture, orfrom any other normally gaseous product mixture described in this patentdocument.

[0029] In another embodiment the H₂/CO ratio of the syngas mixture maybe decreased by removal of hydrogen from the syngas mixture. This can bedone by conventional techniques, such as pressure swing adsorption orcryogenic processes. A preferred option is a separation based onmembrane technology. In the case that hydrogen is removed from thesyngas mixture it may be preferred to apply a two-stage conversion instep (b). The hydrogen is then mixed with the gaseous products of thefirst stage, and together introduced in the second stage. The C₅+selectivity (i.e. the selectivity to hydrocarbons containing 5 or morecarbon atoms, expressed as a weight percentage of the total hydrocarbonproduct) can be improved in this line-up. A portion of the hydrogen maybe used in an optional, additional hydrocracking step in whichespecially the heavier fraction of the hydrocarbons produced in step (b)is cracked, as set out hereinafter.

[0030] Typically the normally gaseous hydrocarbon feed fed to thepartial oxidation of step (a) is completely converted therein.Frequently, the percentage of hydrocarbon feed which is convertedamounts to 50-99%w and more frequently 80-98%w, in particular 85-96%w.

[0031] It is preferred that the heat generated in the partial oxidationis recovered for re-use in the process. For example, the syngas mixtureobtained in step (a) may be cooled, typically to a temperature between100 and 500° C., suitably between 150 and 450° C., preferably between200 and 400° C. Preferably, the cooling is effected in a steam raisingapparatus, such as a boiler, with simultaneous generation of steamtypically of an elevated pressure. Further cooling to temperaturesbetween 30 and 130° C., preferably between 40 and 100° C., may beaccomplished in a conventional heat exchanger, especially in a tubularheat exchanger for example against cooling water or against the feed ledto the reactor, or in an air cooler against air.

[0032] To remove any impurities from the syngas mixture, a guard bed maybe used. Especially to remove all traces of HCN and/or NH₃ specifictypes of active coal may be used. Trace amounts of sulphur may beremoved by an absorption process using iron oxide and/or zinc oxide.

[0033] In step (b) the syngas mixture is converted into the normallyliquid hydrocarbons and normally gaseous hydrocarbons. Suitably at least70%v of the syngas (i.e. the portion of the syngas mixture consisting ofhydrogen and carbon monoxide) fed to step (b), is converted. Preferablyall the syngas fed to step (b) is converted, but frequently 80 to 99%v,more frequently 90 to 98%v is converted. Typically all of the syngasobtained in step (a) is fed into step (b) and more typically all of thesyngas mixture obtained in step (a) is fed into step (b).

[0034] The conversion of step (b) of hydrogen and carbon monoxide intohydrocarbons is well known in the art and it is herein referred to bythe usual term “Fischer-Tropsch synthesis”. Catalysts for use in theFischer-Tropsch synthesis frequently comprise, as the catalyticallyactive component, a metal from Group VIII of the Periodic Table ofElements. Particular catalytically active metals include ruthenium,iron, cobalt and nickel. Cobalt is a preferred catalytically activemetal. Typically, at least a part of the catalytically active metal ispresent in metallic form.

[0035] The catalytically active metal is preferably supported on aporous carrier. The porous carrier may be selected from any of therefractory metal oxides or silicates or combinations thereof known inthe art. Particular examples of preferred porous carriers includesilica, alumina, titania, zirconia, ceria, gallia and mixtures thereof,especially silica and titania.

[0036] The amount of catalytically active metal present in the catalystis preferably in the range of from 3 to 75%w, more preferably from 10 to50%w, especially from 15 to 40%w, relative to the weight of thecatalyst.

[0037] If desired, the catalyst may also comprise one or more metals ormetal oxides as promoters. Suitable metal oxide promoters may be oxidesof elements selected from Groups IIA, IIIB, IVB, VB and VIB of thePeriodic Table of Elements, and the actinides and lanthanides. Inparticular, oxides of magnesium, calcium, strontium, barium, scandium,yttrium. lanthanum, cerium, titanium, zirconium, hafnium, thorium,uranium, vanadium, chromium and manganese are most suitable promoters.Particularly preferred metal oxide promoters for the catalyst used inthe present invention are manganese and zirconium oxide. Suitable metalpromoters may be selected from Groups VIIB and VIII of the PeriodicTable. Rhenium and Group VIII noble metals are particularly suitable,with platinum and palladium being especially preferred. The amount ofpromoter present in the catalyst is suitably in the range of from 0.01to 50%w, preferably 0.1 to 30%w, more preferably 1 to 15%w, relative tothe weight of the catalyst.

[0038] The catalytically active metal and the promoter, if present, maybe deposited on the carrier material by any suitable treatment, such asimpregnation, kneading and extrusion. After deposition of the metal and,if appropriate, the promoter on the carrier material, the loaded carrieris typically subjected to calcination at a temperature generally in therange of from 350 to 750° C., preferably in the range of from 450 to550° C. After calcination, the resulting catalyst may be activated bycontacting the catalyst with hydrogen or a hydrogen-containing gas,typically at a temperature in the range of from 200 to 350° C.Particular forms of catalyst are shell catalysts, in which thecatalytically active metal and the promoter, if present, are positionedin the outer layer of relatively coarse catalyst particles, e.g.extrudates (cf. e.g. U.S. Pat. No. 5,545,674 and the references citedtherein), and catalysts which are present in the form of a powder, e.g.a spray dried powder, suitable for forming a slurry in the liquidreaction medium of the Fischer-Tropsch synthesis (cf. e.g. WO-99/34917).

[0039] Preferably a catalyst is used which comprises cobalt on a titaniacarrier, because such a catalyst is highly efficient in theFischer-Tropsch synthesis in that it provides a high conversion ofsyngas combined with a high C₅+ selectivity, when compared with othercatalysts, thus a low production of the gaseous hydrocarbon products.Preferably, the catalyst contains a further metal selected frommanganese, vanadium, zirconium, rhenium, scandium, platinum andruthenium. Preferably the further metal is manganese or vanadium, inparticular manganese.

[0040] The Fischer-Tropsch synthesis may conveniently and advantageouslybe operated in a single pass mode (“once through”) devoid of any recyclestreams, thus allowing the process to be comparatively simple andrelatively low cost. The process may be carried out in one or morereactors, either parallel or in series. In the case of small hydrocarbonfeedstock streams, the preference will be to use only one reactor.Slurry bubble reactors, ebulliating bubble reactors and fixed bedreactors may be used. In order to minimise the production of gaseoushydrocarbon product, it is preferred to apply fixed bed reactor incombination with a shell type catalyst, or a reactor in combination witha powdery catalyst which is present as a slurry in the liquid reactionmedium.

[0041] The Fischer-Tropsch synthesis may be performed under conventionalconditions known in the art. Typically, the temperature is in the rangeof from 100 to 450° C., preferably from 150 to 350° C., more preferablyfrom 180 to 270° C. Typically, the total pressures is in the range offrom 1 to 200 bara, more preferably from 20 to 100 bara. The GHSV may bechosen within wide ranges and is typically in the range from 400 to10000 Nl/l/h, for example from 400 to 4000 Nl/l/h.

[0042] It is preferred that the heat generated in the Fischer-Tropschsynthesis is recovered for re-use in the process. For example, theFischer-Tropsch reaction mixture may be cooled with simultaneousgeneration of steam typically of an elevated pressure. This may be doneoutside the reactor in which the Fischer-Tropsch synthesis is carriedout, for example in a conventional heat exchanger, or inside thereactor, for example by employing a multi-tubular reactor or by means ofan internal cooling coil. Typically the Fischer-Tropsch reaction mixturemay finally be cooled to a temperature between 40 and 130° C.,preferably between 50 and 100° C., by means of a conventional heatexchanger, especially in a tubular heat exchanger for example againstcooling water or against the feed led to the reactor, or in an aircooler against air.

[0043] The product of step (b), i.e. the product of the Fischer-Tropschsynthesis, is separated in step (c) into the off-gas mixture and afraction comprising the normally liquid hydrocarbon products and,suitably, a fraction comprising the water which is co-produced in theFischer-Tropsch synthesis. This separation may involve distillation andphase separation and it may be carried out using conventional equipment,for example a distillation column or a gas/liquid separator andoptionally a liquid/liquid separator. The off-gas mixture comprisesnitrogen, the normally gaseous hydrocarbon product, and unconvertedhydrogen, carbon monoxide and normally gaseous hydrocarbon feed, if anyof such unconverted components is present. Besides nitrogen, the off-gasmixture may comprise further non-combustible components such as carbondioxide and inert gasses such as helium.

[0044] The pressure of the off-gas mixture is substantially the same asthe pressure prevailing in the Fischer-Tropsch synthesis reactor used instep (b). If the pressure is above 1 bara, it is advantageous to expandthe off-gas mixture, preferably in a turbine using its mechanical energyfor compression purposes. It is preferred to use the mechanical energyof the off-gas mixture for compression of the syngas mixture prior tobeing fed to step (b). This effects that the Fischer-Tropsch synthesisof step (b) is performed at a relatively high pressure which leads to abetter efficiency of the Fischer-Tropsch synthesis, in particular ahigher conversion rate, while the partial oxidation of step (a) isperformed at a relatively low pressure, at which the partial oxidationis more efficient, in terms of a better conversion level. Preferably thepressure increase of the syngas mixture amounts to at least 5 bar, inparticular 10 to 50 bar, more in particular 15 to 40 bar.

[0045] In step (d), at least a part of the off-gas mixture, preferablyat least 90%w, in particular all of the off-gas mixture, is combusted ina steam raising apparatus, generating steam of an elevated pressure.Preferably the off-gas mixture as fed to the steam raising apparatus isslightly above ambient pressure, typically in the range of 1.01 to 5bara, more typically 2 to 4 bara.

[0046] The steam raising apparatus may be conventional equipment, suchas a furnace equipped with heating coils, a boiler or a superheater. Thepressure of the steam generated may be at least 2 bara. Preferably,steam of various pressures is generated simultaneously, for example, alow pressure steam, a medium pressure steam and a high pressure steam.The low pressure steam has a pressure of in the range of 2 to 8 bara,preferably in the range of 3 to 5 bara. The medium pressure steam has apressure of in the range of 8 to 40 bara, preferably in the range of 10to 30 bara. The high pressure steam has a pressure of in the range of 40to 100 bara, preferably in the range of 50 to 80 bara. Preferably, forits efficient use in step (e), the steam produced is superheated steam.Typically the degree of superheating is at least 5° C. For practicalreasons the degree of superheating is at most 100° C. Typically thedegree of superheating is in the range of from 20 to 80° C.

[0047] Customarily, the heating value of a gas is expressedquantitatively by its “lower heating value”. For easy combustion of theoff-gas mixture in the conventional steam raising apparatus, it ispreferred that the off-gas mixture has a composition such that its lowerheating value is in the range of from 3 to 15 MJ/Nm³ (“Nm³” refers tothe gas volume at 0° C., 1 bara). Preferably, the lower heating value ofthe off-gas is in the range of 3.5 to 11 MJ/Nm³, more preferably in therange of 4 to 6 MJ/Nm³. The lower heating value of the off-gas mixturecan be determined experimentally or, if the composition of the off-gasmixture is known, it can be calculated by adding up the weightedcontributions of the lower heating value of the individual components.The lower heating value of the relevant compounds are known to theskilled person.

[0048] In a preferred embodiment of the process the heat of combustionrecovered in step (d) is used together with the heat of reactionrecovered in step (a) and/or the heat of reaction recovered in step (b)for producing steam. For example, superheated high pressure steam havinga pressure in the range of 60 to 65 bara may be produced by heatingwater to form steam using the heat of reaction recovered in step (a),followed by superheating the steam using the heat of combustion of theoff-gas mixture. Alternatively, or preferably simultaneously,superheated medium pressure steam having a pressure in the range of 15to 25 bara may be produced by heating water using the heat of combustionof the off-gas mixture, which steam is then combined with steam of equalpressure produced in the Fischer-Tropsch synthesis reactor of step (b),and subsequently superheated using the heat of combustion of the off-gasmixture. The various steps which involve heating using the heat ofcombustion of the off-gas mixture may be done simultaneously in a singlefurnace in which the off-gas mixture is combusted, by using a pluralityof heating coils.

[0049] In accordance with this invention, at least a part of the steamproduced is used for compressing the air or oxygen enriched air and/orthe normally gaseous hydrocarbon feed. In first embodiment the steam isemployed as a source of shaft power by using for the compression acompressor which is driven by a steam turbine. For example, highpressure steam of 60 to 65 bara, preferably superheated steam of thatpressure, may be employed in the steam turbine for compressing the airor oxygen enriched air used in step (a). Alternatively, or preferablysimultaneously, medium pressure steam of 15 to 25 bara, preferablysuperheated steam of that pressure, may be employed in another steamturbine which drives a compressor compressing the normally gaseoushydrocarbon feed used in step (a). In a second, less preferredembodiment the steam is employed as a source of electrical power byusing for the compression a compressor which is driven electrically andthe electrical power needed is generated using the power of a steamturbine which is driven be steam generated in step (e).

[0050] A further quantity of steam may be used for generating powerwhich is used elsewhere in the process. In particular, it may be usedfor generating electricity, which is used for driving any electricalequipment used in the process, other than the electrically drivencompressors mentioned hereinbefore (if any), such as pumps, air blowers,and other. Sometimes there is a surplus of energy, which may be appliedoutside the process.

[0051] The normally liquid hydrocarbon product as obtained from the step(b) may be transported in liquid form or mixed with any stream of crudeoil without creating problems as to solidification and orcrystallisation of the mixture. It is observed in this respect that instep (b) heavy hydrocarbons products, such as C₁₈₋₂₀₀ hydrocarbons, inparticular C₂₀₋₁₀₀ hydrocarbons, may be coproduced which show a tendencyto solidify as waxy materials, in which case the normally liquidhydrocarbon product becomes more difficult in its handling.

[0052] If this is the case, but also for other reasons, at least part ofthe hydrocarbon product may be subjected to a catalytic hydrocracking,which is known per se in the art. The catalytic hydrocracking is carriedout by contacting the normally liquid hydrocarbon product at elevatedtemperature and pressure and in the presence of hydrogen with a catalystcontaining one or more metals having hydrogenation activity, andsupported on a carrier. Suitable hydrocracking catalysts includecatalysts comprising metals selected from Groups VIB and VIII of thePeriodic Table of Elements. Preferably, the hydrocracking catalystscontain one or more noble metals from group VIII. Preferred noble metalsare platinum, palladium, rhodium, ruthenium, iridium and osmium. Mostpreferred catalysts for use in the hydrocracking stage are thosecomprising platinum. To keep the process as simple as possible, theapplication of a additional catalytic hydrocracking step will usuallynot be a preferred option.

[0053] The amount of catalytically active metal present in thehydrocracking catalyst may vary within wide limits and is typically inthe range of from 0.05 to 5%w, relative to the weight of the catalyst.Suitable conditions for the catalytic hydrocracking are known in theart. Typically, the hydrocracking is effected at a temperature in therange of from about 175 to 400° C. Typical hydrogen partial pressuresapplied in the hydrocracking process are in the range of from 10 to 250bara.

[0054] The catalytic hydrocracking may be carried out before theseparation of step (c), but preferably it is carried out after theseparation of step (c). Additional normally gaseous hydrocarbon products(i.e. hydrocarbon products or a mixture thereof which are gaseous attemperatures between 5 and 30° C. at 1 bara, especially at 20° C. at 1bara) may be formed during the catalytic hydrocracking. Any-off gas ofthe catalytic hydrocracking, comprising any unconverted hydrogen and theadditional normally gaseous hydrocarbon product formed, may be separatedfrom the catalytic hydrocracking reaction product, and added to and/orcombusted with the off-gas mixture in step (d).

[0055] It is an advantage of this invention that the process can becarried out without the need of having available gas which has a highheating value for fueling a gas turbine for power generation, so that itcan advantageously be used as feedstock in the process for theconversion into normally liquid hydrocarbon product. Thus, the normallygaseous hydrocarbon feed can be used completely for conversion purposes.Further, the off-gas mixture may be of a low heating value, which meansthat the partial oxidation and the Fischer-Tropsch synthesis may beoperated at a high efficiency so that the process is performed with ahigh carbon efficiency.

[0056] There is no need to import energy for operating the process fromsources outside the process and/or to install a gas turbine for powergeneration. In the case that energy would be imported from a sourceoutside the process, for example for a reason of convenience, thequantity of energy imported will be less than 50%, preferably less than25%, more preferably less than 10% relative to the energy needed foroperating the process, i.e. the total energy needed to drive the energyconsuming equipment employed in the process, such as heat generatingequipment, compressors, pumps, air blowers, and other.

[0057] The hydrocarbonaceous feed is preferably a normally gaseoushydrocarbon feed, but may also be a solid hydrocarbon feed, e.g. coal,brown coal, peat or organic waste.

[0058] The process may be carried out at a remote location and/oroffshore, for example on a vessel or platform.

1. Process for producing normally liquid hydrocarbon products from ahydrocarbonaceous feedstock, especially from normally gaseoushydrocarbon feed, which process comprises the following steps: (a)partial oxidation of the normally gaseous hydrocarbon feed at elevatedpressure using air or oxygen enriched air as oxidant, to obtain a syngasmixture comprising hydrogen, carbon monoxide and nitrogen; (b)converting hydrogen and carbon monoxide obtained in step (a) into anormally liquid hydrocarbon product and a normally gaseous hydrocarbonproduct; (c) separating from the reaction mixture obtained in step (b)an off-gas mixture comprising nitrogen, normally gaseous hydrocarbonproduct, and unconverted hydrogen, carbon monoxide and normally gaseoushydrocarbon feed, insofar as such unconverted components are present;(d) combusting at least a part of the off-gas mixture in a steam raisingapparatus, producing steam of an elevated pressure; and (e) expandingthe steam produced in step (d) for compressing the air or oxygenenriched air and/or the normally gaseous hydrocarbon feed used in step(a).
 2. A process as claimed in claim 1, wherein the hydrocarbon feed ismethane, natural gas, associated gas or a mixture of C₁₋₄ hydrocarbons,preferably associated gas.
 3. A process as claimed in claim 1 or 2,wherein air is used as the oxidant.
 4. A process as claimed in any ofclaims 1-3, wherein the partial oxidation is a catalytic partialoxidation carried out at a temperature between 800 and 1200° C.,preferably between 850 and 1050° C., and a pressure between 10 and 50bara, preferably between 15 and 40 bara.
 5. A process as claimed in anyof claims 1-4, wherein the conversion of hydrogen and carbon monoxide iscarried out at a temperature in the range of from 150 to 350° C.,preferably from 180 to 270° C., a total pressure in the range of from 1to 200 bara, preferably from 20 to 100 bara, a GHSV in the range from400 to 10000 Nl/l/h, preferably from 400 to 4000 Nl/l/h, and using acatalyst which comprises cobalt on a titania carrier.
 6. A process asclaimed in any of claims 1-5, wherein the off-gas mixture has a lowerheating value in the range of 3.5 to 11 MJ/Nm³, preferably in the rangeof 4 to 6 MJ/Nm³.
 7. A process as claimed in any of claims 1-6, whereinthe heat of reaction produced in step (a) and/or in step (b) isrecovered and the heat of combustion produced in step (d) is usedtogether with the heat of reaction recovered in step (a) and/or the heatof reaction recovered in step (b) for producing steam.
 8. A process asclaimed in any of claims 1-7, wherein the energy used for operating theprocess which is imported from sources outside the process, if any, isless than 25%, preferably less than 10%, relative to the energy neededfor operating the process.
 9. A plant comprising equipment in a line-upsuitable for carrying out a process as claimed in any of claims 1-8,preferably a plant wherein there is no gas turbine installed.
 10. Avessel or a platform which comprises a plant as claimed in claim 9.